This invention relates to a beam splitter for a single lens reflex camera. More particularly, the present invention relates to a beam splitter having the function of directing an imaging light beam toward a viewfinder and in addition, directing part of the imaging light beam to the rear of a mirror (toward a film).
The present invention further relates to a beam splitter which differs from an amplitude dividing type beam splitter using a dielectric material multi-layer film and which may be called the wave surface dividing type or the area type. The present invention also relates to a beam splitter suitable for use in a camera having a device for detecting image information by a digital sensor array after the image information has been transmitted through the beam splitter.
Systems are known in which a light-transmitting portion in the form of a spot or a slit is provided on a quick return mirror for the purpose of metering and part of an imaging light beam is directed to a light-receiving element installed rearwardly of the quick return mirror to effect metering (Japanese Utility Model Publication No. 29793/1969, U.S. Pat. No. 4,081,807, Japanese Laid-open Patent Application No. 119030/1978, etc.).
The reason why the area type beam splitter is used as the quick return beam splitter is that, in the case of a half-mirror, both reflected light and transmitted light are colored or polarized and this leads to deterioration of the viewfinder image and the metering characteristic. The use of the area type beam splitter alleviates the above-noted problems with the color characteristic and polarizing characteristic. But where the size of the light-transmitting portion is large, the shadow thereof is seen through the viewfinder screen or the viewfinder image is sometimes flared by diffraction or the like and therefore, the size of the light-transmitting portion cannot be made very large and the average transmission factor can not be made very great and is suppressed to about 20% or less. Recently, various cameras in which, although not for the purpose of metering, a digital sensor array such as CCD is installed rearwardly of the beam splitter and receives the imaging light beam and takes out the image information to thereby detect the position of the focus have been proposed as shown, for example, in Japanese Laid-open Patent Application No. 18652/1980. Again in such a case, the use of the area type quick return mirror is considered as in Japanese Laid-open Patent Application No. 105205/1980 because of the ease of manufacture thereof.
However, applicant has found that where image information is to be detected by a digital sensor array having a pitch of 10.mu., for example, where the digital sensors are effectively installed on the predetermined imaging plane and at two points before and behind the plane to effect focus detection, low-pass filter means is required between an object to be photographed and the sensors. Applicant has also shown that the MTF (modulation transfer function) of the imaging light beam reaching the sensor surface is controlled by the pattern shape of the transmitting portion of the area type quick return beam splitter to achieve the purpose, and has also proposed a specific pattern.
In the case of the focus detection camera by applicant (Japanese Laid-open Patent Application No. 18652/1980), use is made of a blur detecting system and three digital line sensors installed on the predetermined imaging plane and equidistant before and behind the plane, to receive image information and to detect the in-focus state from the comparison between the blurs at the respective positions. Accordingly, if the maner of blurring on the sensor arrays placed equidistant before and behind the predetermined imaging plane differ from each other at the in-focus state, it will adversely affect the accuracy of focus detection. As is well known in the art, in the case of an imaging lens of small FNo such as F1.4 or F1.2, the front blur and the rear blur often differ from each other during the opening under the influence of spherical aberration. For such a case, a method of providing a specially configured opening (see FIG. 1 of the accompanying drawings) at the rear of a half-mirror for the purpose of reducing the rate at which the light passed through the marginal portion of the lens reaches the sensor surface has been devised. In FIG. 1, one half of the light beam from the imaging lens (in FIG. 1, indicated by the on-axis light ray for convenience) 1 is reflected by a whole surface half-mirror 4 and a light beam 3 travelling toward the viewfinder and the remaining one half of the light beam passes through the half-mirror 4 and enters a focus detecting optical system 9. A part of the back of the half-mirror 4, namely, that part through which a light beam does not pass, is provided with a light-absorbing coating material layer 5 so as to prevent ghost from being created by back side reflection. Immediately beind the light-transmitting portion, there is installed a thin mask 6 having an opening approximately of a semicircular shape and the light after being transmitted through the mask is reflected by a sub-mirror 8 and enters a light beam 2 into the system 9 for dividing the light beam into three light beams. The function of the mask having the opening approximately of a semicircular shape will now be described with reference to FIG. 2 of the accompanying drawings.
Referring to FIG. 2, the mask 6 has a semicircular opening 10 and this shape is such that when a light beam of F1.4 enters along the optical axis 12, the cross-sectional shape of the light beam on the mask surface becomes an ellipse 11 and accordingly, the marginal portion of the light beam is scarcely passed.
The function of the mask 6 can also be provided by carrying out such evaporation on the quick return beam splitter of FIG. 1 that the transmission factor is reduced in the direction away from the optical axis, but this has the disadvantage that the process is cumbersome.